General purpose distributed data parallel computing using a high level language

ABSTRACT

General-purpose distributed data-parallel computing using a high-level language is disclosed. Data parallel portions of a sequential program that is written by a developer in a high-level language are automatically translated into a distributed execution plan. The distributed execution plan is then executed on large compute clusters. Thus, the developer is allowed to write the program using familiar programming constructs in the high level language. Moreover, developers without experience with distributed compute systems are able to take advantage of such systems.

CLAIM OF PRIORITY

This application is a continuation application of U.S. patentapplication Ser. No. 12/368,231, entitled “GENERAL PURPOSE DISTRIBUTEDDATA PARALLEL COMPUTING USING A HIGH LEVEL LANGUAGE,” filed Feb. 9,2009, published as US 2010/0205588 on Aug. 12, 2010 and issued as U.S.Pat. No. 9,110,706 on Aug. 18, 2015, and incorporated herein byreference in its entirety.

BACKGROUND

One of the most challenging problems in the field of computing today ishow to allow a wide variety of software developers to computeeffectively on large amounts of data.

Parallel processing is one technique that has been employed forincreasing the efficiency of computing on large amounts of data.Traditionally, parallel processing refers to the concept of speeding-upthe execution of a program by dividing the program into multiplefragments that can execute concurrently, each on its own processor. Aprogram being executed across n processors might execute n times fasterthan it would using a single processor. The terms concurrently andparallel are used to refer to the situation where the period forexecuting two or more processes overlap in time, even if they start andstop at different times. It is possible to perform parallel processingby connecting multiple computers in a network and distributing portionsof the program to different computers on the network.

However, many software application developers are not experienced withparallel processing. Therefore, it can be difficult for them to write anapplication that can take advantage of parallel processing. Moreover, itis often difficult to divide an application program in such a way thatseparate processors can execute different portions of a program withoutinterfering with each other. There has been a great deal of researchperformed with respect to automatically discovering and exploitingparallelism in programs which were written to be sequential. The resultsof that prior research, however, have not been successful enough formost developers to efficiently take advantage of parallel processing ina cost effective manner.

SUMMARY

The technology described herein pertains to general-purpose distributeddata-parallel computing using a high-level language. Data parallelportions of a sequential program that is written by a developer in ahigh-level language are automatically translated into a distributedexecution plan. The distributed execution plan is then executed in adistributed compute system. Thus, the developer is allowed to write theprogram using familiar programming constructs in the high levellanguage. Moreover, developers without experience with distributedcompute systems are able to take advantage of such systems.

In one embodiment, an expression is accessed from a program running at aclient machine. An execution plan is automatically generated forexecuting the expression in parallel at nodes of a compute cluster. Theexecution plan is provided to an execution engine that controls parallelexecution of the expression in the compute cluster.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one embodiment of hardware organization ofa distributed execution engine, as well as software that runs thereon.

FIG. 2 is a block diagram of a computing machine that can be used toimplement one embodiment of the nodes depicted in FIG. 1.

FIG. 3 is an example of a directed graph.

FIG. 4 is a logical view of the system depicted in FIG. 1.

FIG. 5 depicts a flowchart describing one embodiment of a processperformed by a job manager when executing code on the distributedexecution engine of FIG. 1.

FIG. 6 depicts one embodiment of software and data structures used fordata-parallel processing.

FIG. 7 depicts one embodiment of a process for executing data-parallelportions of the application program in a distributed compute system.

FIG. 8 is one embodiment of a process for compiling expressions from auser application into a distributed execution plan.

FIG. 9 depicts one embodiment of static and dynamic optimizations of agraph.

DETAILED DESCRIPTION

The technology described herein pertains to general-purpose distributeddata-parallel computing using a high-level language. A developer createsa sequential program in a high level language (“application program”).The application program may be a hybrid program that has code thatexecutes on a client machine and data-parallel portions that areexecuted in parallel in a compute cluster. For example, thedata-parallel portions that execute in the compute cluster might rankweb pages, whereas the code that executes on the client might display agraphic to a user based on the ranking. In some implementations, adistributed execution provider automatically translates thedata-parallel portions of the application program into a distributedexecution plan. The distributed execution plan is then executed on nodesin a compute cluster. In some embodiments, the distributed executionplan includes an execution plan graph (“EPG”) and code for the verticesof the EPG (“vertex code”). The distributed execution provider may alsoserialize data objects that are referenced in the application programand needed for execution of the vertex code in the compute cluster.Moreover, serialization code that allows access to data types of theobjects in the application program is generated. The serialized dataobjects and serialization code may be considered to be part of thedistributed execution plan. In some embodiments, the distributedexecution provider generates additional code, such as code that is usedto facilitate optimizing execution in the compute cluster.

In some embodiments, the overall system can be considered to be brokeninto three distinct pieces: 1) an application layer, 2) an executionengine, and 3) storage. The application layer includes both theapplication that the developer wrote and the distributed executionprovider that automatically generates the distributed execution plan.The execution engine receives the execution plan and manages parallelexecution in the compute cluster. This separation may allow theapplication layer to interoperate with a variety of different types ofexecution engines, as well as a variety of different types of storagelayers.

In some embodiments, the distributed execution provider provides theautomatically generated distributed execution plan (e.g., EPG, vertexcode, serialized data objects and serialization code) to an executionengine for execution in the compute cluster. Thus, the execution enginemay be a separate program from the distributed execution provider thatgenerated the distributed execution plan. FIG. 1 is a block diagram ofan architecture for a suitable execution engine that is implemented as atree-structure network 10 having various sub-networks within thetree-structure connected via switches. The execution engine can be usedto cause the data-parallel portions of the application program toexecute in the compute cluster. However, note that the data-parallelportions of the application program can be executed by a differentexecution engine than the example described herein. In other words, thecode that is automatically generated (e.g., vertex code) can be executedby a different execution engine.

Sub-network 12 includes Job Manager 14 and Name Server 16. Sub-network12 also includes a set of switches 20, 22, . . . , 24. Each switchconnects sub-network 12 with a different sub-network. For example,switch 20 is connected to sub-network 30 and switch 24 is connected tosub-network 40. Sub-network 30 includes a set of switches 32, 34, . . ., 36. Sub-network 40 includes a set of switches 42, 44, . . . , 46.Switch 32 is connected to sub-network 50. Switch 42 is connected tosub-network 60. Sub-network 50 includes a set of computing machines 52,54, . . . , 56. Sub-network 60 includes a set of computing machines 62,64, . . . , 66. Computing machines 52, 54, . . . , 56 and 62, 64, . . ., 66 (as well as other computing machines at the bottom levels of thehierarchy of the tree-structured network) make up the cluster ofmachines that form the distributed execution engine. Although FIG. 1shows three levels of hierarchy, more or fewer than three levels can beused. In another embodiment the network may not be tree-structured, forexample it could be arranged as a hypercube.

The automatically generated vertex code is executed as a parallelprocessing job (hereinafter referred to as a “job”) that is coordinatedby Job Manager 14, which is a process running on a dedicated computingmachine or on one of the computing machines in the compute cluster. Jobmanager 14 is responsible for instantiating a job's dataflow graph,scheduling processes on nodes in the compute cluster to cause the vertexcode to execute, providing fault-tolerance by re-executing failed orslow processes, monitoring the job and collecting statistics, andtransforming the job dataflow graph (or simply “job graph”) dynamicallybased on callbacks in order to optimize execution. Name Server 16 isused to report the names (or other identification information such as IPAddresses) and position in the network of all of the computing machinesin the cluster. There is a simple daemon (or service) running on eachcomputing machine in the cluster which is responsible for creatingprocesses on behalf of Job Manager 14.

FIG. 2 depicts an exemplary computing device 100 for implementing thevarious computing machines of the cluster (e.g., machines 52, 54, . . ., 56 and 62, 64, . . . , 66), Job Manager 14 and/or Name Server 16. Inits most basic configuration, computing device 100 typically includes atleast one processing unit 102 and memory 104. Depending on the exactconfiguration and type of computing device, memory 104 may be volatile(such as RAM), non-volatile (such as ROM, flash memory, etc.) or somecombination of the two. Processing unit 102 may be a single core, dualcore or other form of multiple core processing unit. This most basicconfiguration is illustrated in FIG. 2 by line 106.

Additionally, device 100 may also have additionalfeatures/functionality. For example, device 100 may also includeadditional storage (removable and/or non-removable) including, but notlimited to, magnetic disk, optical disks or tape. Such additionalstorage is illustrated in FIG. 2 by removable storage 108 andnon-removable storage 110. Computer storage media includes volatile andnonvolatile, removable and non-removable media implemented in any methodor technology for storage of information such as computer readableinstructions, data structures, program modules or other data. Memory104, removable storage 108 and non-removable storage 110 are allexamples of computer (or processor) readable storage media. Such mediaincludes, but is not limited to, RAM, ROM, EEPROM, flash memory or othermemory technology, CD-ROM, digital versatile disks (DVD) or otheroptical storage, magnetic cassettes, magnetic tape, magnetic diskstorage or other magnetic storage devices, or any other medium which canbe used to store the desired information and which can accessed bydevice 100. Any such computer storage media may be part of device 100.

Device 100 may also contain communications connection(s) 112 that allowthe device to communicate with other devices via a wired or wirelessnetwork. Examples of communications connections include network cardsfor LAN connections, wireless networking cards, modems, etc.

Device 100 may also have input device(s) 114 such as keyboard, mouse,pen, voice input device, touch input device, etc. Output device(s) 116such as a display/monitor, speakers, printer, etc. may also be included.All these devices (input, output, communication and storage) are incommunication with the processor.

The technology described herein can be implemented using hardware,software, or a combination of both hardware and software. The softwareused is stored on one or more of the processor readable storage devicesdescribed above to program one or more of the processors to perform thefunctions described herein. In alternative embodiments, some or all ofthe software can be replaced by dedicated hardware including customintegrated circuits, gate arrays, FPGAs, PLDs, and special purposecomputers.

In some embodiments, a distribution execution provider analyzes portionsof the user application and automatically generates a file thatdescribes a directed graph (also referred to herein as an EPG) and codefor vertices of the directed graph. As an example, the file thatdescribes the directed graph could be an XML file. Job Manager 14 willbuild a job graph based on the file that describes the directed graphand manage the distribution of the vertex code to the various computenodes of the distributed compute cluster. FIG. 3 provides one example ofa block diagram of a directed graph that represents a system that readsquery logs gathered by an Internet search service, extracts the querystrings, and builds a histogram of query frequencies sorted byfrequency. In the example of FIG. 3, the directed graph is acyclic;however, the directed graph could be cyclic.

In some embodiments, a job's external input and output files arerepresented as vertices in the graph even though they do not execute anyprogram. Typically, for a large job, a single logical “input” is splitinto multiple partitions which are distributed across nodes in thesystem as separate files. Each of these partitions can be represented asa distinct input vertex. In some embodiments, there is a graphconstructor which takes the name of a distributed file and returns agraph made from a sequence of its partitions. The application willinterrogate its input graph to read the number of partitions at runtimein order to generate the appropriate replicated graph. For example, FIG.3 shows six partitions or files 202, 204, 206, 208, 210 and 212 of thelog created by the Internet search service.

The first level of the hierarchy of the graph of FIG. 3 includes vertexcode (Co) for implementing vertices 220, 222, 224, 226, 228 and 230.Herein, the code for a particular vertex may also be referred to as a“vertex program” or simply a “vertex.” As already stated, a distributedexecution provider of some embodiments automatically generates thisvertex code from expressions in the application program running on theclient machine. The vertex code (Co) reads its part of the log files,parses the data to extract the query strings, sorts the query stringbased on a hash of the query string, and accumulates the total countsfor each query string. Although eight vertices are shown (220, 222, . .. 230), more or fewer than eight vertices can be used. In oneembodiment, there will be one vertex at this level for each partition ofthe log. Each of the vertices will output a set of hashes representingthe query strings and a total count for each hash. This information willthen be sent to an appropriate aggregator (Ag) vertex, depending on thehash.

FIG. 3 shows three vertices 242, 244 and 246 implementing the aggregator(Ag). The potential set of queries will be broken up into three buckets,with one subset of hashes being sent to aggregator 242, a second subsetof hashes being sent to aggregator 244, and a third subset of hashesbeing sent to aggregator 246. In some implementations, there will bemore or fewer than three aggregators. Each of the vertices 220-230 willbe in communication with all of the aggregators to send data to theappropriate aggregator based on the hash. The aggregators 242, 244 and246 will aggregate all of the counts for each query based on datareceived from vertices 220-230. Each of the aggregators 242, 244 and 246will report its data to (Su) vertex 250, which will combine the sums forall of these various queries and store those sums in results file 256.As can be seen, vertices 220-230 access data in parallel and can beexecuted in parallel. Similarly, aggregators 242-246 can also beexecuted in parallel. Thus, Job Manager 14 will distribute the vertexcode to maximize efficiency for the system.

In one embodiment, a job utilizing the technology described herein hastwo levels of abstraction. At a first level of abstraction, the overallstructure of the job is determined by the communication flow. Thiscommunication flow is the directed graph where each vertex is a processand edges represent data channels. In some embodiments, the directedgraph is automatically generated based on analysis of the applicationprogram running on the client. The directed graph is automaticallymapped onto physical resources by the execution engine. The second levelof abstraction is the vertex code which implement the vertices.

In some embodiments, every vertex program deals with its input andoutput through the channel abstraction. As far as the body of vertexprograms is concerned, channels transport objects. This ensures that thesame vertex program is able to consume its input either from disk orwhen connected to a shared memory channel—the last case avoidsserialization/deserialization overhead by passing the pointers to theobjects directly between producer and consumer. Note that other channelsimplementations including, but not limited to, TCP pipes and HTTPconnections are possible.

In some implementations, the base class for vertex programs suppliesmethods for reading any initialization parameters which were set duringgraph construction and transmitted as part of the vertex invocation.These include a list of string arguments and an opaque buffer into whichthe program may serialize arbitrary data. When a vertex program is firststarted but before any channels are opened, the runtime calls a virtualinitialization method on the base class. This method receives argumentsdescribing the number of input and output channels connected to it.

In one implementation, the input and output channels are opened beforethe vertex program starts. In another implementation channels are openedas needed, which requires fewer resources on the channel endpoint fromwhich data originates and which may speed-up execution. In some cases,channels are opened in a random order to minimize resource contention.Any error at this stage causes the vertex program to report the failureand exit. This will trigger Job Manager 14 to try to recreate themissing input. In other embodiments, other schemes can be used. When allof the channels are opened, a vertex Main routine is called and passedchannel readers and writers for all its inputs and outputs respectively.The readers and writers may have a blocking interface to read or writethe next item which suffices for most simple applications. There may bea method on the base class for inputting status which can be read by themonitoring system, and the progress of channels may be automaticallymonitored. An error reporting interface allows that vertex program tocommunicate a formatted string along with any additionalapplication-defined metadata. The vertex program may exit before readingall of its inputs. A process which contains a long pipeline of vertexprograms connected via shared memory channels and ending, for example,with a “Head” vertex will propagate the early termination of Head allthe way back to the start of the pipeline and exit without reading anyunused portion of its inputs. In other embodiments, other schemes can beused.

FIG. 4 provides a logical view of the system depicted in FIG. 1 and howthat system makes use of the vertex code 302. FIG. 4 shows Job Manager14 connected to network system 352, which can be the network 10 ofFIG. 1. Also connected to network system 352 are Name Service 316 and aset of computing machines 362, 364 and 366. Although FIG. 4 only showsthree computing machines, it is possible to have fewer than threecomputing machines or more than three computing machines. In someembodiments there could be thousands of computing machines. Eachcomputing machine has a process daemon (PD) running Job Manager 14 willcause the various process daemons to run various vertices (e.g.,vertices 372, 374, 376), which are in communication with the data plane320, which is an arbitrary data transport mechanism used to implementchannels (e.g., files, TCP pipes, or shared memory channels). JobManager 14 includes vertex code 302, library 354, job graph 356, VertexQueue 358, and Node Queue 360.

Library 354 provides a set of code to enable Job Manager 14 to create ajob graph 356, build the job graph 356, and execute the job graph 356across the distributed execution engine. In one embodiment, library 354can be embedded in C++ using a mixture of method calls and operatoroverloading. In one embodiment, library 354 defines a C++ base classfrom which all vertex programs inherit. Each such program has a texturalname (which is unique within an application) and a static “factory”which knows how to construct it. A graph vertex may be created bycalling the appropriate static program factory. Any requiredvertex-specific parameter can be set at this point by calling methods onthe program object. The parameters are then marshaled along with theunique vertex name (referred to herein as a unique identification-UID)to form a simple closure which can be sent to a remote process orexecution. Every vertex program is placed in a stage. In someimplementations, a stage is created by replicating one vertex. In alarge job, all the vertices in a level of hierarchy of the graph mightlive in the same stage; however, this is not required. In otherembodiments, other schemes can be used.

The first time a vertex program is executed on a computer, its binary issent from the Job Manager 14 to the appropriate process daemon (PD). Thevertex program can be subsequently executed from a cache. In someembodiments, all vertices in a job share the same binary, which allowsfor efficient caching because vertex binaries sent for one stage can bereused by other stages. Job Manager 14 can communicate with the remotevertices, monitor the state of the computation, monitor how much datahas been read, and monitor how much data has been written on itschannels. Legacy executables can be supported as vertex programs byconnecting the legacy executable with named pipes to a stub whichredirects the data from the pipes to channels.

Job Manager 14 keeps track of the state and history of each vertexprogram in the job graph 356. A vertex program may be executed multipletimes over the length of the job due to failures, and certain policiesfor fault tolerance. In one implementation, each execution of the vertexprogram has a version number and a corresponding execution record whichcontains the state of the execution and the versions of the predecessorvertices from which its inputs are derived. In one aspect, eachexecution names its file-based output channel uniquely using its versionnumber to avoid conflicts when multiple versions execute simultaneously.In one implementation, each vertex executes in a separate isolated“sand-box.” Therefore, multiple versions of the same vertex do not clashbecause each one uses a separate sand-box. One implementation ofsand-boxes is to use separate root directories. However, more compleximplementations, based on virtual machines are possible. The sand-boxesmay be managed by the process daemons. If the entire job completessuccessfully, then each vertex program selects one of its successfulexecutions and renames the output files to their correct final forms.

When all of a vertex program's input channels become ready, a newexecution record is created for the vertex program in the “Ready” stateand gets placed in Vertex Queue 358. A disk based channel is consideredto be ready when the entire file is present. A channel which is a TCPpipe or shared memory FIFO is ready when the predecessor vertex has atleast one execution record in the “Running” state.

Each of the vertex's channels may specify a “hard constraint” or a“preference” listing the set of computing machines on which it wouldlike to run. The constraints are attached to the execution record whenit is added to Vertex Queue 358 and they allow the application writer torequire that a vertex be collocated with a large input file, and ingeneral that the Job Manager 14 preferentially run computations close totheir data.

When a Ready execution record is paired with an available computer ittransitions to the Running state (which may trigger vertices connectedto its parent via pipes or FIFOs to create new Ready records). While anexecution is in the Running state, Job Manager 14 receives periodicstatus updates from the vertex. On successful completion, the executionrecord enters the “Completed” state. If the vertex execution fails, therecord enters the “Failed” state, which may cause failure to propagateto other vertices executing in the system. A vertex that has failed willbe restarted according to a fault tolerance policy. If every vertexsimultaneously has at least one Completed execution record, then the jobis deemed to have completed successfully. If any vertex is reincarnatedmore than a set number of times, the entire job has failed.

Files representing temporary channels are stored in directories managedby the process daemon and are cleaned up after job completion.Similarly, vertices are killed by the process daemon if their parent jobmanager crashes.

FIG. 5 depicts a flowchart describing one embodiment of a processperformed by Job Manager 14 when executing vertex code 302 on thedistributed execution engine of FIG. 1. In step 402, Job Manager 14creates the job graph 356 based on the EPG and vertex code 302. In oneembodiment, the EPG is a description of an execution plan, such as adescription written in XML. Thus, Job Manager 14 may create the jobgraph 356 from an XML description.

In step 404, Job Manager 14 receives a list of nodes from Name Server16. Name Server 16 provides Job Manager 14 with the name (oridentification) of each node within the network as well as the positionof each node within the tree-structured network. In many embodiments, anode is a computing machine. In some embodiments, a computing machinemay have more than one node.

In step 406, Job Manager 14 determines which of the nodes are available.A node is available if it is ready to accept another program (associatedwith a vertex) to execute. In one implementation, Job Manager 14 querieseach process daemon to see whether it is available to execute a program.In one implementation, Job Manager 14 assumes that all machines listedby the NS are available. If Job Manager 14 cannot connect to a PD (or ifa PD fails to often), then Job Manager 14 marks the PD as unusable. JobManager 14 may dispatch several copies of each vertex to a set ofprocess daemons chosen according to a scheduling algorithm. In step 408,Job Manager 14 populates all of the available nodes into Node Queue 360.In step 410, Job Manager 14 places all the vertices that need to beexecuted into Vertex Queue 358. In step 412, Job Manager 14 determineswhich of the vertices in Vertex Queue 358 are ready to execute. In oneembodiment, a vertex is ready to execute if all of its inputs areavailable.

In step 414, Job Manager 14 sends instructions to the process daemons ofthe available nodes to execute the vertices that are ready to beexecuted. Job Manager 14 pairs the vertices that are ready with nodesthat are available, and sends instructions to the appropriate nodes toexecute the appropriate vertex. In step 416, Job Manager 14 sends thecode for the vertex to the node that will be running the vertex code, ifthat code is not already cached on the same machine or on anothermachine that is local (e.g., in same sub-network). In most cases, thefirst time a vertex is executed on a node, its binary will be sent tothat node. After executing the binary, that binary will be cached. Thus,future executions of that same code need not be transmitted again.Additionally, if another machine on the same sub-network has the codecached, then the node tasked to run the code could get the program codefor the vertex directly from the other machine on the same sub-networkrather than from Job Manager 14. After the instructions and code areprovided to the available nodes to execute the first set of vertexes,Job Manager 14 manages Node Queue 360 in step 418 and concurrentlymanages Vertex Queue 358 in step 420.

Managing node queue 418 includes communicating with the various processdaemons to determine when there are process daemons available forexecution. Node Queue 360 includes a list (identification and location)of process daemons that are available for execution. Based on locationand availability, Job Manager 14 will select one or more nodes toexecute the next set of vertices. Steps 412-416 may be repeated untilall vertices have been run.

Further details of execution engines can be found in U.S. PublishedPatent Application 2008/0082644, entitled “Distributed ParallelComputing;” U.S. Published Patent Application 2008/0098375, entitled“Runtime Optimization of Distributed Execution Graph;” and U.S.Published Patent Application 2008/0079724, entitled “DescriptionLanguage for Structured Graphs;” all of which are all herebyincorporated by reference for all purposes.

FIG. 6 depicts one embodiment of software and data structures used fordata-parallel processing. In general, FIG. 6 depicts a client machine602 and a distributed compute system 604. The client machine 602 may beimplemented with computer device 100 depicted in FIG. 2. The client 602is running an application program 610 and a distributed execution plan614 that extracts expressions 612 (or expression trees) from theapplication program 610. Based on the expressions 612, the distributedexecution provider 614 generates an execution plan graph (“EPG”) 622,vertex code 302, and serialized data objects and serialization code 616,which are each provided to the distributed compute system 604. In thisexample, Job Manager 14 distributes the vertex code 302 and serializedobjects and serialization code 616 to compute nodes 52-66.

Note that the application program 610 may be a sequential program thathas code that executes on the client 602 in addition to thedata-parallel portions that execute in the distributed compute system604. For example, the data-parallel code might perform a page-rank ofweb pages, whereas the code that executes on the client 602 mightpresent the page rank statistics to a user in a graphical userinterface. Thus, the application program 610 may be thought of as a“hybrid” program. Note that in some conventional systems two separateprograms would need to be written to accomplish what application program610 performs. For example, a first program might be written in alanguage such as SQL to perform database queries and second programmight be written in a language such as C to perform functions at theclient device. Moreover, in some embodiments, the developer does notneed to be concerned over which variables are local to the client 602and which are remote because the distributed execution provider 614takes care of this.

The application program 610 may have both declarative and imperativeoperations. The application program 610 may include traditionalstructuring constructs such as functions, modules, and libraries, andexpress iteration using standard loops. In some embodiments, thedistributed execution plan employs a fully functional, declarativedescription of the data-parallel components, which enables sophisticatedrewritings and optimizations such as those traditionally employed byparallel databases.

In one implementation, the application program 610 is written in theLINQ (Language INtegrated Queries) programming language. A LINQ programis a sequential program composed of LINQ expressions. A LINQ program isa Microsoft .NET Framework component that adds native data queryingcapabilities to .NET languages. The .NET framework is a softwareframework that is available with several Windows® operating systems thatare available from Microsoft corporation of Redmond, Wash. A LINQprogram can be debugged using standard .NET development tools. Theapplication program 610 is not limited to LINQ nor is it limited to the.NET Framework.

FIG. 7 depicts one embodiment of a process 700 for executingdata-parallel portions of the application program 610 in a distributedcompute system 604. FIG. 6 will be referred to when discussed theprocess 700. In step 702, a user application 610 executes on the clientmachine 602. In one embodiment, the user application 610 is written bythe developer in a high level language. In one embodiment, theapplication program 610 creates one or more expressions 612 duringruntime. However, the actual execution of the expression 612 may bedeferred until later in the process 700.

In one implementation, the expression 612 is based on classes providedby a .NET library. In one aspect, the expression 612 is base on .NET“Expression” classes. A .NET Expression class is in the namespaceSystem.Linq.Expression. There are numerous subclasses, such asBinaryExpression, ConstantExpression, UnaryExpression, LambdaExpression,MemberAssignment, etc. For example, a expression 612 may be implementedas a tree of expression classes with each node in the tree being anoperator. Child nodes may show inputs to operators. As a specificexample, the addition of two constants may be represented as a tree witha root of “BinaryExpression” and two leaf nodes containing the constantexpressions. Thus, as previously discussed an expression 612 might alsobe referred to as an expression tree.

In step 704, the user application 610 initiates data parallel execution,which may result the expression 612 being passed to the distributedexecution provider 614. In one aspect, the user application 610 makes acall in order to initiate data parallel execution. However, it is notrequired that the user application 610 make call to initiate dataparallel execution. In one aspect, data parallel execution is initiatedin response to the user application 610 attempting to enumerate a valuefor an expression 612. When the user application 610 attempts toenumerate a value for the expression 612, data parallel execution isinitiated to compute the value.

In step 706, the distributed execution provider 614 compiles theexpression 612 into a distributed execution plan 622. Briefly, step 706may include the decomposition of the expression 612 intosub-expressions. Each sub-expression corresponds to a vertex. Step 706may also include the automatic generation of the vertex code, as well asstatic data for the vertices. Further, serialization code may beautomatically generated for the data types needed to execute at theremote computer nodes.

The following briefly describes step 706. Further details are discussedbelow in the discussion of process 800 of FIG. 8. As previouslydiscussed, in some implementations, the expressions 612 are based on theExpression class of a .NET library. In one aspect, the distributedexecution provider 614 manipulates and transforms the expression 612 andbreaks it into pieces. In one aspect, each piece is used to generate C#code, which is the vertex code 302. Note that data structuresrepresented by the expressions 612 may be similar to syntax trees thatare used by compilers to represent the code during the compilationprocess.

In step 708, the distributed execution provider 614 invokes a JobManager 14. In one embodiment, the Job Manager 14 executes behind afirewall. In step 710, Job Manager 14 creates a job graph 356 using thedistributed execution plan 622 that was generated in step 704. JobManager 14 schedules and spawns the vertices as resources becomeavailable in the distributed compute system 604. In step 712, each ofthe vertices executes the code 302 that was generated in step 704. Thecompute nodes have access to input tables 633 to make computations. Theinput tables 633 are data that is being processed by the userapplication 610. Some of the input tables 633 can be based on the resultof a previous computation performed by the distributed compute system604 for the user application 610. However, the data in the input tables633 can be based on some other external computation. Note that the inputtables 633 may be composed of partitions that reside on differentmachines and that each partition can have replicas on differentmachines. In step 714, the job completes and the results are output tothe distributed compute system output tables 652.

In step 716, Job Manager 14 terminates, returning control back to thedistributed execution provider 614. In step 718, the distributedexecution provider 614 creates local table objects 654 encapsulating theoutput of execution in the distributed compute system 604. These localobjects 654 may then be used as inputs to subsequent expressions 612 inthe user application program 610. In one implementation, local tableobjects 654 are fetched to the local context only if explicitlyde-referenced.

In step 720, control returns to the user application program 610. Theuser application 610 has access to the local table objects 654. In oneimplementation, an iterator interface allows the user application 610 toread the local table objects 654 as .NET objects. However, there is norequirement of using .NET objects.

In step 722, the application program 610 may generate subsequentexpressions 612, which may be executed by repeating steps 704-720.

FIG. 8 is one embodiment of a process 800 for compiling an expression612 from a user application 610 into a distributed execution plan.Process 800 is one implementation of step 706 of process 700. In step802, an execution plan graph (EPG) is generated from an expression 612.Step 802 occurs when the distributed execution provider 614 receivescontrol after the application program 610 initiates parallel processing.The distributed execution provider 614 converts the raw expression 612into an execution plan graph (EPG) 622, where each vertex is an operatorand edges represent its inputs and outputs. The EPG 622 may be relatedto a conventional database query plan; however, the EPG 622 mayencompass computations that are not easily formulated as “queries.” TheEPG 622 may be a directed graph, which may or may not be acyclic. Theexistence of common sub-expressions and operators like “Fork” means thatEPGs cannot always be described by trees. In some implementations, theEPG 622 is a “skeleton” that is written in a language such as XML. Forexample, the EPG 622 may be a skeleton of the job data-flow graph 356that will be executed by the execution engine.

In step 804, static optimizations of the EPG 622 are performed. In oneimplementation, the distributed execution provider 614 appliesterm-rewriting optimizations on the EPG 622. In one embodiment, each EPGnode is replicated at run time to generate a “stage,” which may bedefined as a collection of vertices running the same computation ondifferent partitions of a dataset. In one implementation, the optimizerannotates the EPG 622 with metadata properties. For edges of the EPG622, these annotations may include the data type and the compressionscheme, if any, used after serialization. In one implementation, thedata types are .NET data types. For nodes of the EPG 622, theannotations may include details of the partitioning scheme used, andordering information within each partition. The output of a node, forexample, might be a dataset that is hash-partitioned by a particularkey, and sorted according to that key within each partition. Thisinformation can be used by subsequent OrderBy nodes to choose anappropriate distributed sort algorithm. In one aspect, the propertiesare seeded from the LINQ expression tree and the input and outputtables' metadata, and propagated and updated during EPG rewriting.

Propagating these properties may be more difficult than for aconventional database. The difficulties stem from the much richer datamodel and expression language used to create the application program610. Consider one of the simplest operations: input.Select(x=>f(x)). Iff is a simple expression, e.g. x.name, then it is straightforward forthe distributed execution provider 614 to determine which properties canbe propagated. However, for arbitrary fit is very difficult to determinewhether this transformation preserves the partitioning properties of theinput.

Fortunately, the distributed execution provider 614 can usually inferproperties in the application programs 610 typical users write.Partition and sort key properties are stored as expressions, and it isoften feasible to compare these for equality using a combination ofstatic typing, static analysis, and reflection. In one embodiment, asimple mechanism is provided that allows users to assert properties ofan expression 612 when it is difficult or impossible to determine theproperties automatically. Further details of static optimizations arediscussed below.

In step 806, the vertex code 302 and static data for the vertices aregenerated. While the EPG 622 encodes all the required information, it isnot necessarily a runnable program. In one embodiment, dynamic codegeneration automatically synthesizes LINQ code to be run at thevertices. The generated code may be compiled into a .NET assembly thatis shipped to cluster computers at execution time. The sub-expression ina vertex may be built from pieces of the overall EPG 622. In someimplementations, the EPG 622 is created in the original clientcomputer's execution context, and may depend on this context in twoways: (1) The expression 612 may reference variables in the localcontext. These references are eliminated by partial evaluation of thesub-expression at code-generation time. For primitive values, thereferences in the expressions 612 may be replaced with the actualvalues. Object values are serialized to a resource file which is shippedto computers in the cluster at execution time. (2) The expression 612may reference .NET libraries. In this case, .NET reflection may be usedto find the transitive closure of all non-system libraries referenced bythe executable, which are shipped to the cluster computers at executiontime.

In step 808, serialized objects and serialization code 616 are generatedfor required data types. As previously mentioned, the user application610 can be thought of as a hybrid program that has code for executing atthe client 602 and code that is executed in parallel in the distributedcompute system 604. It may be that the user application 610 refers to alocal data object that is needed by the vertex code 302. Theserialization code may be bundled with the vertex code 302 and shippedto compute nodes. The serialization code allows the compute nodes toread and write objects having the required data types. The serializedobjects are provided to the vertices because the vertex code 302references those objects. Note that the developer is not required todeclare which data is local and which data is remote. The serializationcode 616 allows data to be passed in the channels between the vertices.This serialization code 616 can be much more efficient than standard.NET serialization methods since it can rely on the contract between thereader and writer of a channel to access the same statically knowndatatype.

In step 810, the distributed execution provider 614 generates code forperforming dynamic optimizations. Generating code for dynamicoptimization is discussed below.

In various embodiments, the distributed execution provider 614 performsboth static and dynamic optimizations. The static optimizations may begreedy heuristics or cost-based optimizations. The dynamic optimizationsare applied during job execution and may consist in rewriting the jobgraph depending on run-time data statistics. In various implementations,the optimizations are sound in that a failure to compute propertiessimply results in an inefficient, though correct, execution plan.

In one embodiment, the static optimizations are conditional graphrewriting rules triggered by a predicate on EPG node properties. Staticoptimizations may be focused on minimizing disk and network I/O. Someimportant optimizations include the following. However, many other typesof optimizations can be performed.

Pipelining: Multiple operators may be executed in a single process. Thepipelined processes may themselves be expressions 612 and can beexecuted by an existing single-computer LINQ implementation.

Removing redundancy: The distributed execution provider 614 removesunnecessary hash- or range-partitioning steps.

Eager Aggregation: Since re-partitioning datasets is expensive,down-stream aggregations are moved in front of partitioning operatorswhere possible.

I/O reduction: Where possible, the distributed execution provider 614takes advantage of TCP-pipe and in-memory FIFO channels instead ofpersisting temporary data to files. In one embodiment, data is bydefault compressed before performing a partitioning in order to reducenetwork traffic. Users are allowed to manually override compressionsettings to balance CPU usage with network load if the optimizer makes apoor decision.

In one embodiment, API hooks are used to dynamically mutate the jobgraph 356 as information from the running job becomes available. Forexample, the distributed execution provider 614 provides “callback code”to Job Manager 14. This callback code is added to the job graph 356.During runtime, this callback code causes information to be gathered andused to dynamically mutate the job graph 356. The callback code may alsoperform the dynamic optimizations based on the gathered information.

In one implementation, the mutation is based on aggregation. Aggregationgives a major opportunity for I/O reduction since it can be optimizedinto a tree according to locality. Data may be aggregated first at thecomputer level, next at the rack level, and finally at the clusterlevel. The topology of such an aggregation tree can only be computed atrun time, since it is dependent on the dynamic scheduling decisionswhich allocate vertices to computers. The distributed execution provider614 may use techniques discussed in U.S. Published Patent Application2008/0098375, entitled “Runtime Optimization of Distributed ExecutionGraph, which has already been incorporated herein by reference in itsentirety.

In one embodiment, dynamic data partitioning is used. Dynamic datapartitioning sets the number of vertices in each stage (i.e., the numberof partitions of each dataset) at run time based on the size of itsinput data. Conventional databases usually estimate dataset sizesstatically, but these estimates can be very inaccurate. As one example,the estimates may be inaccurate in the presence of correlated queries.In one embodiment, dynamic hash and range partitions are supported. Forrange partitions both the number of partitions and the partitioning keyranges are determined at run time by sampling the input dataset.

The following example for sorting a dataset d illustrates many of thestatic and dynamic optimizations available. Different strategies areadopted depending on d's initial partitioning and ordering. FIG. 9 showsthe evolution of an OrderBy node O 902 in a complex case, where d is notalready range-partitioned by the correct sort key, nor are itspartitions individually ordered by the key. The transformation ofOrderBy node O 902 to graph 904 is static. The transformation of graph904 to graph 906 and then graph 906 to graph 908 are both dynamic, basedon information learned at runtime.

Referring now to graph 904, first the dataset is re-partitioned. The DSstage performs deterministic sampling of the input dataset. The samplesare aggregated by a histogram vertex H, which determines the partitionkeys as a function of data distribution (load-balancing the computationin the next stage). The D vertices perform the actual repartitioning,based on the key ranges computed by H. Next, a merge node M interleavesthe inputs, and a S node sorts them. M and S are pipelined in a singleprocess, and communicate using iterators.

The number of partitions in the DS+H+D stages of graph 906 is chosen atrun time based on the number of partitions in the preceding computation.The number of partitions in the M+S stages of graph 908 is chosen basedon the volume of data to be sorted.

As previously discussed, some embodiments use the LINQ framework. One ofthe benefits of using the LINQ framework is that other systems that usethe same or similar constructs can be leveraged. For example, PLINQ,which allows code to be run within each vertex in parallel on amulti-core server, can be leveraged. PLINQ is described in, “A QueryLanguage for Data Parallel Programming,” J. Duffy, Proceedings of the2007 Workshop on Declarative Aspects of Multicore Programming, 2007,which is hereby incorporated by reference for all purposes. PLINQattempts to make the process of parallelizing a LINQ program astransparent as possible. PLINQ employs the iterator model since it isbetter suited to fine-grain concurrency in a shared-memorymulti-processor system. Because both PLINQ and embodiments of thepresent invention use expressions composed from the same LINQconstructs, their functionality may be combined. In some embodiments,vertices execute LINQ expressions, and in general the addition by thecode generator of some embodiments of a single line to the vertex'sprogram triggers the use of PLINQ, allowing the vertex to exploit allthe cores in a cluster computer.

In some implementations, interoperation with a LINQ-to-SQL system allowsvertices to directly access data stored in SQL databases. Running adatabase on each cluster computer and storing tables partitioned acrossthese databases may be much more efficient than using flat disk filesfor some applications. Application programs 610 can use “partitioned”SQL tables as input and output in some embodiments. The distributedexecution provider 614 of some embodiments identifies and ships somesubexpressions to the SQL databases for more efficient execution.

Further, a single-computer LINQ-to-Objects implementation allowsapplications programs 610 to be run on a single computer for testing onsmall inputs under the control of a debugger before executing on a fullcluster dataset. In one implementation, the debugger is a part of theVisual Studio® development system, which is available from MicrosoftCorporation of Redmond, Wash. Debugging a distributed application is anotoriously difficult problem. Distributed application jobs may be longrunning, processing massive datasets on large clusters, which could makethe debugging process even more challenging. One embodiment is amechanism to run applications on a single computer with verysophisticated support from the .NET development environment. Once anapplication is running on the cluster, an individual vertex may fail dueto unusual input data that manifests problems not apparent from asingle-computer test. One aspect uses a deterministic-replay executionmodel, which allows re-execution of such a vertex in isolation with theinputs that caused the failure, and the system includes scripts to shipthe vertex executable, along with the problematic partitions, to a localcomputer for analysis and debugging.

Performance debugging is a much more challenging problem. In someembodiments, programs report summary information about their overallprogress. However, if particular stages of the computation run moreslowly than expected, or their running time shows surprisingly highvariance, it may be necessary to investigate a collection of disparatelogs to diagnose the issue manually. The centralized nature of the jobmanager makes it straightforward to collect profiling information toease this task.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims. It is intended that the scopeof the invention be defined by the claims appended hereto.

We claim:
 1. A computer readable storage hardware device having storedthereon computer executable instructions which, when executed on aprocessor, cause the processor to: execute a sequential applicationprogram comprising a data parallel portion that includes an expression,wherein the sequential application program is written in a high-levellanguage and comprises both imperative operations and declarativeoperations; access the expression from a portion of the sequentialapplication program that comprises a declarative operation; based on theexpression, generate an execution plan graph having vertices and datachannels between the vertices, the execution plan graph for executingthe expression in parallel, including causing the processor to break theexpression into a plurality of sub-expressions, each of thesub-expressions is a vertex in the execution plan graph; generate vertexcode for the vertices of the execution plan graph; generate code thatallows data to be passed in the data channels; access results ofexecuting the execution plan graph, the vertex code and the code thatallows data to be passed in the data channels; and execute a portion ofthe sequential application program that comprises an imperativeoperation to present the results in a user interface.
 2. The computerreadable storage hardware device of claim 1, wherein the computerexecutable instructions further cause the processor to: execute theexecution plan graph, the vertex code, and the code that allows data tobe passed in the data channels to cause the results to be produced. 3.The computer readable storage hardware device of claim 1, wherein thecomputer executable instructions further cause the processor to: providethe execution plan graph, the vertex code, and the code that allows datato be passed in the data channels to a job manager to execute theexecution plan graph and to produce the results.
 4. The computerreadable storage hardware device of claim 1, wherein the computerexecutable instructions further cause the processor to: execute theexecution plan graph to cause parallel execution of the expression. 5.The computer readable storage hardware device of claim 1, wherein theexecution plan graph is a directed graph.
 6. The computer readablestorage hardware device of claim 1, wherein the vertices of theexecution plan graph represent processes.
 7. The computer readablestorage hardware device of claim 6, wherein the data channels transportobject between the processes.
 8. The computer readable storage hardwaredevice of claim 1, wherein the computer executable instructions furthercause the processor to: serialize data objects that are referenced inthe sequential application program, wherein the vertex code referencesthose data objects.
 9. The computer readable storage hardware device ofclaim 8, wherein the computer executable instructions further cause theprocessor to: provide the execution plan graph, the vertex code, thecode that allows data to be passed in the data channels, and theserialized data objects to a job manager to execute the execution plangraph and to produce the results.
 10. A method comprising: executing asequential application program comprising a data parallel portion thatincludes an expression, wherein the sequential application program iswritten in a high-level language and comprises both imperativeoperations and declarative operations, wherein the sequentialapplication program executes on a computing machine having a processor;accessing, by the processor, the expression from a portion of thesequential application program that comprises a declarative operation;based on the expression, automatically generating an execution plangraph having vertices and data channels between the vertices, theexecution plan graph for executing the expression in parallel, includingcausing the processor to break the expression into a plurality ofsub-expressions, each of the sub-expressions is a vertex in theexecution plan graph; automatically generating vertex code for thevertices of the execution plan graph by the processor; automaticallygenerating serialization code that allows data to be passed in the datachannels; accessing, by the processor, results of executing theexecution plan graph, the vertex code and the serialization code; andexecuting, by the processor, a portion of the sequential applicationprogram that comprises an imperative operation to present the results ina user interface.
 11. The method of claim 10, further comprising:executing the execution plan graph, the vertex code, and theserialization code to cause the results to be produced.
 12. The methodof claim 10, further comprising: executing the execution plan graph tocause parallel execution of the expression.
 13. The method of claim 10,wherein the execution plan graph is a directed graph.
 14. The method ofclaim 10, wherein the vertices of the execution plan graph representprocesses.
 15. A device comprising: a processor readable storagehardware device; a processor coupled to the processor readable hardwarestorage device, wherein the processor readable hardware storage deviceincludes instructions that cause the processor to: execute a sequentialapplication program comprising a data parallel portion that includes anexpression, wherein the sequential application program is written in ahigh-level language and comprises both imperative operations anddeclarative operations; access the expression from a portion of thesequential application program that comprises a declarative operation;based on the expression, generate an execution plan graph havingvertices and edges between the vertices, the execution plan graph forexecuting the expression in parallel, including causing the processor tobreak the expression into a plurality of sub-expressions, each of thesub-expressions is a vertex in the execution plan graph; generate vertexcode for implementing the vertices of the execution plan graph; generateserialization code that allows data to be passed between the vertex codeof different vertices joined by an edge; access results of executing theexecution plan graph, the vertex code, and the serialization code; andexecute a portion of the sequential application program that comprisesan imperative operation to present the results in a user interface. 16.The device of claim 15, wherein the instructions further cause theprocessor to: execute the execution plan graph, the vertex code, and theserialization code to cause the results to be produced.
 17. The deviceof claim 15, wherein the instructions further cause the processor to:provide the execution plan graph, the vertex code, and the serializationcode-to a job manager to execute the execution plan graph to produce theresults.
 18. The device of claim 15, wherein the instructions furthercause the processor to: execute the execution plan graph to causeparallel execution of the expression.
 19. The device of claim 15,wherein the execution plan graph is a directed graph.
 20. The device ofclaim 15, wherein each vertex is an operator and the edges represent itsinputs and outputs.